Accessing portions of a dynamic array in C? - c

Question

I know, another dynamic array question, this one is a bit different though so maybe it'll be worth answering. I am making a terrain generator in C with SDL, I am drawing 9 chunks surrounding the screen, proportional to the screen size, that way terrains can be generated easier in the future.
This means that I have to be able to resize the array at any given point, so I made a dynamic array (at least according to an answer I found on stack it is) and everything SEEMS to work fine, nothing is crashing, it even draws a single tile....but just one. I am looking at it and yeah, sure enough it's iterating through the array but only writing to one portion of memory. I am using a struct called Tile that just holds the x, y, w, and h of a rectangle.
This is the code I am using to allocate the array
Tile* TileMap = (Tile*)malloc(0 * sizeof(Tile*));
int arrayLen = sizeof(TileMap);
TileMap = (Tile*)realloc(TileMap, (totalTiles) * sizeof(Tile));
arrayLen = sizeof(totalTiles * sizeof(Tile));
The totalTiles are just the number of tiles that I have calculated previously are on the screen, I've checked the math and it's correct, and it even allocates the proper amount of memory. Here is the code I use to initialize the array:
//Clear all elements to zero.
for (int i = 0; i < arrayLen; i++)
{
Tile tile = {};
TileMap[i] = tile;
}
So what's weird to me is it is considering the size of a tile (16 bytes) * the totalTiles (78,000) is equaling 4....When I drill down into the array, it only has one single rect in it that gets cleared as well, so then when I go calculate the sizes of each tile:
//Figure out Y and heights
for (int i = startY; i <= (startY*(-1)) * 2; i += TILE_HEIGHT)
{
TileMap[i].y = i * TILE_HEIGHT;
TileMap[i].h = TILE_HEIGHT;
//Figure out X and widths
for (int j = startX; j <= (startX*(-1)) * 2; j += TILE_WIDTH)
{
TileMap[i].x = i * TILE_WIDTH;
TileMap[i].w = TILE_WIDTH;
}
}
*Side note, the startX is the negative offset I am using to draw chunks behind the camera, so I times it by -1 to make it positive and then time it by two to get one chunk in front of the camera
Alright, so obviously that only initializes one, and here is the render code
for (int i = 0; i < totalTiles; i++)
{
SDL_Rect currentTile;
currentTile.x = TileMap[i].x;
currentTile.y = TileMap[i].y;
currentTile.w = TileMap[i].w;
currentTile.h = TileMap[i].h;
SDL_RenderDrawRect(renderer, &currentTile);
}
free(TileMap);
So what am I doing wrong here? I mean I literally am just baffled right now...And before Vectors get recommended in place of dynamic arrays, I don't really like using them and I want to learn to deal with stuff like this, not just implement some simple fix.

Answer 1

Lots of confusion (which is commonplace with C pointers).
The following code doesn't provide expected answer :arrayLen = sizeof(totalTiles * sizeof(Tile));
totalTiles * sizeof(Tile) is not even a type, I'm surprised it compiles at all. Edit : See molbnilo comment below. so it provides the size of the return type.
Anyway, proper answer should be :
arrayLen = totalTiles;
Because that's what you need in your next loop :
//Clear all elements to zero.
for (int i = 0; i < arrayLen; i++)
{
Tile tile = {};
TileMap[i] = tile;
}
You don't need the size of the table, you need its number of elements.
There are other confusions in your sample, they don't directly impact the rest of the code, but better correct them :
Tile* TileMap = (Tile*)malloc(0 * sizeof(Tile*)); : avoid allocating a size of 0.
int arrayLen = sizeof(TileMap); : no, it's not the arrayLen, just the size of the pointer (hence 4 bytes on 32-bits binaries). Remember TileMap is not defined as a table, but as a pointer allocated with malloc() and then realloc().